Backing up a minute, this changes the dynamic previously suggested considerably. We can flee the identified SoD monster, but we have no ability to swap out the cleric's spell load for appropriate protections. I guess we can buy or commission scrolls, but that definitely adds time, so if we're under any time pressure, that may not be an option. Alternatively, if the cleric has the protective spells on his spontaneous list, then we're in little danger if we can ID the monster. One of the more effective Clerics I've seen in play had two combat-ready domains, and Domain Spontaneity twice. He could swap out (up to his Turn Undead attempts) any spell for a combat spell (domain spell) or (unlimited) curative spell. So he could load up with situational spells - if the situation looked like it would not come up, they got swapped.
So, IMC, "retreat and look for the most useful spells in the spellbook and memorize them tomorrow and come back" is not really a viable tactic. However "retreat and cast whatever buff spells we do know (Death Ward included), then come back a minute or two later" works fine. "Retreat and buy a wand of Death Ward" is a last resort.
If the monster is simply wandering around open space, there is often not much reason to kill it anyway, so the party may just want to avoid taking on a Medusa if they're concerned about the risk. If the party wanted to get rid of the monster to prevent it from killing harmless innocents in the area, then they might come back. If there's time pressure, they might have to charge in. But at least they'll probably charge in knowing what the risk is and how to mitigate it (if possible). Again, most SoD monsters below CR 10 allow a character to avoid making a save completely if he so chooses. Another IMC houserule variable is action points. Simply knowing what your saving throw is against influences your decision to action point the save.
An LG Cleric may well believe in "an eye for an eye; a life for a life". Again, it's OK to kill the Orc to prevent him killing the farmer, but it's not OK to kill him to Raise the farmer he killed? Either one results in a dead Orc and a live farmer.
Life for a life = No. That does not, in my view, fall within the LG alignment. LN neutral character perhaps. And yes, one might question how different the two orc scenarios are, but the same thing comes up in real "value of life" debates. For example, some people draw a strong line between physician-assisted suicide and euthanasia, while others say that both are absolutely wrong and others say that both are justifiable. Legally, we draw lines between murder, manslaughter, and homicide in self-defense, as well as numerous subdivisions, and we have capital punishment. Context matters, to some people some of the time at least.
I consider the ability to discern between right and wrong to be a key to "a soul". In a D&D world, they are commodities for lower planar beings, who need to turn these soul to evil to "win". How do they corrupt those who cannot make moral judgements? Where do their souls go?
I would say that such souls go wherever your cosmology dictates. Many souls die without having ever made a moral decision. What's the infant mortality rate in D&D? To me, the D&D "soul" concept is pretty broad.
But have we not established that the value of a life (to a good character) far outstrips that of a diamond?
Of course. I'm merely identifying a mechanical precedent for how my houseruled spell works.
That second example is scary - how many fans throw themselves on the sword hoping to be "the one" that his hero accepts. Not a pro or a con to the structure, just an observation.
People are that way. "Fan" is derivative of "fanatic". The implications
are scary. If one of our modern heroes was in a severe accident and needed an organ transplant...
To that guaranteed evil - "respect for life" seems like it could easily have a different connotation when returning a fallen person is actually possible.
Valid enough. We all have sociocultural influences on our own moral views. It's hard to dissociate from those when talking about D&D morality. If you're running a game in pre-Columbian Central America (or Maztica, perhaps?), sacrifices and trading of lives are probably accepted without a second thought.
One problem with ethics is that, even where there is a pretty easy answer in theory, the answer in practice is commonly a lot tougher to implement.
The kinds of ethics I typically deal with are either in biomedical/psychological research or clinical medicine. Modern medicine (like magic, almost) often gives us scenarios where it is difficult to even intellectualize right and wrong, let alone implement it.